The long term objective of this research program is to characterize the ion conductance properties of Muller cell membrane and to determine the role that Muller cells play in electroretinogram (ERG) generation and in the regulation of extracellular K+ levels in the retina. Studies conducted during the previous grant period demonstrated that the amphibian Muller cell membrane is almost exclusively permeable to K+ and that 84 to 95% of total Muller cell conductance is located in the endfoot process of the cell. Specific aims for the forthcoming grant period include the following: 1) Determine whether the Muller cell endfeet of mammals have high K+ conductance properties similar to those of amphibians. 2) Localize precisely and determine the specific membrane conductance of the high K+ conductance region of the Muller cell endfoot. 3) Investigate the recently discovered voltage-sensitive conductance properties of the Muller cell membrane. 4) Determine the pharmacological properties of Muller cell K+ channels. 5) Determine whether the endfeet of mammalian retinal astrocytes have high K+ conductance. 6) Conduct direct experimental tests to determine whether Muller cells generate components of the ERG and contribute to the regulation of K+ levels within the retina. 7) Study Muller cell function using computer simulations of K+ dynamics and current generation in the retina. Muller cell membrane properties and function will be studied in dissociated cell, retinal slice and eyecup preparations of amphibian and mammalian species using intracellular, patch-clamp and ion-selective microelectrode recording techniques. The studies outlined in this proposal will clarify the role that Muller cells play in the generation of the ERG, which is an important clinical tool used in diagnosing many retinal disorders including diabetic retinopathy and retinitis pigmentosa. These studies will also elucidate the role that Muller cells play in regulating K+ levels within the retina. Disruption of such regulation may be related to pathological retinal conditions.